This kind of accordion menus are mainly used for websites that are specifically designed for mobiles like smartphones. They take up less horizontal space and display the optimum information provided in the menu which can be collapsed and expanded giving access to various links in a portal or website.

Also used in responsive web designs where the entire navigation bar spans accross the screen in a tab or a PC screen whereas the same website is seen scaled down and succumbing itself to the accordion menu when we compress the browser window or open the site in a mobile browser with smaller screensize.